磁性材料的特性.pptx

CHAPTER16MAGNETICPROPERTIESOFMATERIALS16.1EffectonMagneticFieldbyMaterials16.2Magnetization16.3VectorHandItsCircuitalTheorem16.4MagneticShielding16.1EffectonMagneticFieldbyMaterialsTheinteractionbetweenmagneticfieldsandmaterialscanbeTheinteractionbetweenmagneticfieldsandmaterialscanbeobservedexperimentally.Themostsimplemethodistousealongobservedexperimentally.Themostsimplemethodistousealongstraightwithvacuumorairinsidefirst(Fig.16.1(a)andmeasurethestraightwithvacuumorairinsidefirst(Fig.16.1(a)andmeasurethemagneticfieldinitwhencurrentIisflowinginthecoils.magneticfieldinitwhencurrentIisflowinginthecoils.Nextfillthesolenoidwithsomematerialandthenmeasureagainthemagneticfieldinitwhenthesamecurrentisledinthecoils(Fig.16.1(a).LetB0andBbethemagneticfieldmeasuredinthetwocasesrespectively.Experimentalresultsshowthatthetwodifferfromeachotherandcanrelatedas16.2MagnetizationInanatom,theelectronsorbitaroundthenucleusandhavealsospin.Inanatom,theelectronsorbitaroundthenucleusandhavealsospin.Thenucleusalsohasspin.ThesemotionallfromequivalentcircularThenucleusalsohasspin.Thesemotionallfromequivalentcircularelectriccurrents.Asawholethesemotionsofchargesinamoleculeelectriccurrents.Asawholethesemotionsofchargesinamoleculemaybeconsideredasconstituteasinglemicroscopiccircularcurrentmaybeconsideredasconstituteasinglemicroscopiccircularcurrentcalledcalledmolecularcurrentmolecularcurrentiwithiwithmolecularmagneticmomentmolecularmagneticmomentm.m.Example16.1AlongstraightsolenoidofnturnperunitlengthisfilledExample16.1Alongstraightsolenoidofnturnperunitlengthisfilledinsidewithuniformmaterialofrelativepermeability.Findthesurfaceinsidewithuniformmaterialofrelativepermeability.Findthesurfaceboundcurrentdensityonthesurfaceofthematerialinthesolenoid.boundcurrentdensityonthesurfaceofthematerialinthesolenoid.16.3VectorHandItsCircuitalTheorem16.4MagneticShieldingWhenabockofferromagneticWhenabockofferromagneticmaterialisputinamagneticmaterialisputinamagneticfield.Sinceitsisverylarge.field.Sinceitsisverylarge.ThesurfaceboundcurrentThesurfaceboundcurrentproducedbymagnetizationisproducedbymagnetizationisverylarge.Thesuperpositionofverylarge.Thesuperpositionofthemagneticfieldproducedbythemagneticfieldproducedbytheseboundcurrentsonthetheseboundcurrentsontheoriginalexternalfieldwouldoriginalexternalfieldwouldchargethedistributionofthechargethedistributionofthelaterdrastically.Pictorially,thelaterdrastically.Pictorially,theresultantfieldlineswouldberesultantfieldlineswouldbeconcentratedorcollectedintheconcentratedorcollectedintheferromagneticmaterialasferromagneticmaterialasshowninFig.16.4.showninFig.16.4.Iftheferromagneticmaterialsismadetohaveahollowinterior,suchasanironbox,themagneticfieldlineswouldbecollectedintheironwallasshowninFig.16.5Inthehollowspaceinthebox,themagneticfieldwouldbeveryweak.Thuswhensomethingisputintheclosedbox.Itwillnot(oronlyslightly)beaffectedbyexternalmagneticfields.Thisistheprincipleofmagneticshieldingusingironbox.SUMMARY